1. Field of the Invention
This invention relates to an improvement in a fluid coking process.
2. Description of the Prior Art
Fluid coking is a well known process which may be carried out with or without recycle of a heavier portion of the fluid coking zone effluent. As is well known in the art, the fluid coking process, as shown, for example, in U.S. Pat. No. 2,881,130, which is hereby incorporated by reference, uses a fluid coking vessel and an external heating vessel. A fluid bed of solids, preferably coke particles produced by the process, having a size in the range from about 40 to about 1000 microns, is maintained in the coking zone by the upward passage of fluidizing gas, usually steam, injected at a superficial velocity typically between 0.3 and 5 feet per second. The temperature in the fluid coking bed is maintained in the range of about 850 to about 1400.degree. F., preferably between 900.degree. and 1200.degree. F. by circulating solids (coke) to the heating vessel and back. The heavy oil to be converted is injected into the fluid bed and upon contact with the hot solids undergoes pyrolysis evolving lighter hydrocarbon products in vapor phase, including normally liquid hydrocarbons, and depositing a carbonaceous residue (coke) on the solids. The turbulence of the fluid bed normally results in substantially isothermal reaction conditions and thorough and rapid distribution of the heavy injected oils. The feed rate and temperature are controlled to maintain the bed in a fluidized state. Product vapors, after removal of entrained solids, are withdrawn overhead from the coking zone and sent to a scrubber and fractionator for cooling and separation. The end boiling point of distillate fractions obtained from the process is usually about 1050.degree. F. to about 1200.degree. F. and the remaining heavy ends are usually recycled to extinction.
The conventional fluid coking process is generally carried out to maximize the yields of lower boiling normally liquid hydrocarbon products, particularly components boiling in the gas oil range, e.g. 430.degree. to 1050.degree. F. The fluid coking process may also be used as a feed pretreatment to effect partial demetallization and to lower the Conradson carbon content of the feed without maximizing conversion to liquid products. Thus, a low severity fluid coking process would be desirable for pretreatment of carbonaceous feeds prior, for example, to hydroconversion, hydrodesulfurization, catalytic cracking and other processes which operate more effectively on low metals feed or low Conradson carbon feed.
An improved low severity fluid coking process has now been found in which the feed is first treated in a dense fluidized bed and the effluent of the dense fluidized bed is treated in a transferline zone.